This invention relates generally to optical systems and more particularly to devices used for the transmission of laser radiation therethrough.
It is generally known that the application of laser radiation to crystals beyond certain energy thresholds result in both surface damage on the radiation entrance and/or exit faces of the crystals and bulk damage within the interior of the crystal. In transparent dielectrics, surface damage is due to electron avalanche processes, induced by localized electric fields which occur in response to nanosecond incident laser pulses having a power density above a characteristic threshold, and in response to picosecond incident laser pulses having an energy density above a characteristic threshold. For dielectrics which are self-absorbing, or which contain impurity inclusions that can absorb incident radiation, the above-noted surface and bulk damage is compounded by heating of the crystal structure. Primary causes of the induced surface damage are surface structural defects (such as microscopic grooves created by polishing materials) which produce localized intensifications of the electric field leading to avalanche breakdown. Generally, when no voids or impurity inclusions exist in the bulk of a non-absorbing material, radiation damage thresholds on solid surfaces are lower than those within the bulk material. Thus, for most materials, the surface damage threshold provides the limit to the power or energy density of a beam which may be applied to a crystal.
To overcome surface damage to a significant degree, the entrance and exit faces of certain crystalline materials may be highly polished so that the dimensions of any grooves or inclusions are 100 Angstroms or less, thereby raising the surface damage thresholds to those for the bulk material and consequently, permitting higher energy incident radiation. This approach to the surface damage problem has been experimentally confirmed using ultra-fine polishing techniques, e.g., ion-beam or bowl-feed methods. See D. W. Fradin and M. Bass, Applied Physics Letters, 22, 157 (1973). However, laser-induced surface damage is especially severe in a number of non-linear crystalline materials such as proustite (Ag.sub.3 AsS.sub.3) and cinnabar (HgS), which are relatively soft and correspondingly difficult and expensive to polish to an ultra-fine degree.
These uniaxial crystalline materials may be characterized by two different principal indices of refraction, the ordinary index of refraction, n.sub.o, and the extraordinary index of refraction, n.sub.e. Such crystals, which are sufficiently non-linear and birefringent (e.g. proustite), may be used to produce an output light signal having a frequency equal to the difference between the frequencies associated with two incident linearly and orthogonally polarized light signals. See, for example, F. Zernike and J. E. Midwinter, Applied Non-linear Optics, John Wiley, New York 1973. In that application, the first input beam is controlled to have a polarization perpendicular to the plane formed by the crystal's optical axis and the direction of beam propagation. This beam is known as the "ordinary ray" within the interior of the crystal and is subject to the ordinary index of refraction, n.sub.o. The ordinary index of refraction is invariant with the angular orientation of the propagation vector of the incident light beam relative to the crystal optical axis.
The second input beam is controlled to have a polarization in the plane formed by the crystal's optical axis and the direction of beam propagation. This latter beam is known as the "extraordinary ray" and is subject to the extraordinary index of refraction. The extraordinary index of refraction is a function of the angular orientation of the propagation vector of the incident light beam with respect to the crystal's optical axis.
In order to maximize the energy transfer from two incident linearly and orthogonally polarized light beams of differing frequency to the resultant difference frequency light beam generated therein, the crystal's optical axis must be oriented relative to the incident beams so that the input and output beams are phase matched and that constructive interference occurs in the crystal interior region (see Zernike and Midwinter, cited above, Chapters 2 and 3). Accordingly, in order to generate a substantial intensity beam at a specific infrared wavelength, for example, two appropriately separated (in frequency) linearly and orthogonally polarized light beams may be applied to a sufficiently non-linear and birefringent crystal at the specific orientation relative to the crystal optical axis required for phase matching. The required orientation may be determined from the wavelength dispersion characteristic for the crystal in accordance with cited Zernike and Midwinter reference.
The intensity of the resultant difference frequency beam is directly dependent upon and limited by the intensity of incident beams which are applied to the crystal. Although known ultra-fine polishing techniques are effective to raise the surface damage thresholds to those of bulk material for some of the non-linear materials, the costs associated with such techniques set severe practical limitations to their use in the production of laser devices for laser systems.
Another known technique to minimize the surface damage problem in non-linear laser materials is to apply a solid coating to the surface of the crystal wherein the solid coating has an index of refraction substantially matched to the crystal. In order for this technique to be effective, the solid coating must smooth out the imperfections in the crystal surfaces by filling in the pits or grooves in order to provide a reduction in the induced local electric fields. However, experimental results indicate that a thin coating, either vapor-deposited or sputtered on a crystal surface, tends to assume the shape of the surface, thereby retaining its defects and in some cases emphasizing their effects. However, if the thin film is more damage resistant than the crystal material, the surface damage is somewhat reduced, and an effective rise in the surface damage threshold may be attained. In fact, sapphire coatings on LiNbO.sub.3 have been observed to raise surface damage thresholds by a factor of approximately 2.5, but even with this improvement, the surface damage threshold is still substantially below bulk damage thresholds.
Alternatively, instead of the thin film, polished flats can be optically contacted to the faces of the crystal, for example, by bonding fused-silica to LiNbO.sub.3 with the result that an approximate four-fold increase in the damage threshold is observed for radiation at 1.06 m. See W. D. Fountain, L. M. Osterink and G. A. Massey, ASTM-NBC Symposium on Laser Damage in Materials: 1971, A. J. Glass and A. H. Guenther, Editors, NBC Spec. Publ. 356 (1971).
Thus, although these index of refraction matching solid coatings do provide increases on the surface damage thresholds to a measurable degree, none of the approaches have been successful in raising that threshold to the level observed for the bulk material.
It is an object of this invention to provide optical devices wherein the surface damage threshold is substantially the same as the damage threshold for the bulk of the crystal.
It is the further object of this invention to provide devices for transmission of high power laser radiation therethrough.
Other and more specific objects of the invention will become apparent from the description and figures which follow.